Applicant""s disclosure is directed to an improved method of synchronizing a hub radio and a remote radio. Specifically, applicant""s disclosure is directed to an improved method of synchronizing plural radios in a point-to-multipoint Broadband Wireless Access (BWA) communication system which uses Time Division Multiple Access (TDMA).
In a conventional communication system, each transceiver or radio may have a modem for modulating and demodulating the signals communicated between the transceivers. In order for a demodulator to correctly demodulate a received signal, the frequency of the receiving transceiver must be synchronized to the frequency of the transmitting transceiver. The process of synchronizing the frequency of one transceiver with the frequency of another transceiver is known as ranging. The ranging process typically occurs when a transceiver is first powered up, and then periodically to ensure it is synchronized with other transceivers in the system. Typically, the frequency of one of the transceivers is held constant while the frequency of the other transceivers is adjusted to synchronize the frequencies.
In a conventional point-to-multipoint communication system, in which a hub radio communicates with a plurality of remote radios utilizing a single frequency, it is common to use Time Division Multiple Access (TDMA) to prevent interference between the radios. It is crucial that the hub and the remote radios are synchronized to the same frequency. In principal, each of the radios transmit at the same frequency. However, due to frequency source uncertainties, which may vary from radio to radio, the signals transmitted to the hub radio may arrive with varying frequency offsets. If the frequency offset for a given remote radio is greater than the frequency tolerance of the hub radio, then the frequency offset is outside of the frequency range of capture of the hub radio and therefore the hub radio will be unable to synchronize with that remote radio. For a point-to-multipoint communication system, it is common to maintain the frequency of the hub radio constant, and adjust the frequency of each of the remote radios to match the frequency of the hub radio.
In one prior art ranging process, the hub radio will transmit a synchronization signal to the remote radios. Each remote radio will then perform a complex quantitative sampling of the received synchronizing signal and derive the frequency of the hub radio from the synchronizing signal. The remote radio will then adjust is frequency to match that of the hub radio. The remote will send a ranging signal at the newly adjusted frequency and this iterative process between the remote radio and the hub radio continues until the radios are synchronized or some predetermined time limit is reached. This type of ranging process requires that each remote have the ability to perform complex signal processing in order to derive the frequency of the hub radio from the synchronizing symbol.
In another prior art ranging process, the remote radio initiates the process by providing a ranging signal to the hub radio. The hub radio samples the received ranging signal and estimates the transmission frequency of the remote radio. The hub radio determines the frequency offset between the respective frequencies of the hub radio and the remote radio and determines a frequency offset correction necessary to adjust the frequency of the remote radio to match the frequency of the hub radio. The hub radio transmits a synchronizing signal to the hub radio which provides the remote radio with the frequency offset correction. The remote radio receives the frequency offset correction from the synchronizing signal and adjusts its frequency accordingly. The remote radio will transmit another ranging signal at the adjusted frequency and this iterative process between the remote radio and the hub radio continues until the radios are synchronized or some predetermined time limit is reached. This type of ranging process requires that the hub radio have the capability to perform complex signaling processing and waveform analysis of the ranging signals in order to determine the frequency offset for each of the remote radios.
Thus, the prior art ranging systems typically require complex and expensive signaling processing equipment. These prior art ranging systems are generally unable to synchronize radios where the frequency offset between the radios exceeds 64 kHz. In Broadband Wireless Access (BWA) systems, it is not uncommon to see frequency offsets in excess of 64 kHz, and therefore the prior art synchronization systems are not adequate for use in a BWA communication systems.
The present disclosure is directed to a synchronizing method which does not require complex signaling processing and is able to synchronize at frequency offsets much greater than the prior art systems.
Accordingly, it is an object of the present invention to provide a novel method and of achieving synchronization between radios in a point-to-multipoint TDMA communication system.
It is another object of the present invention to provide a novel method of increasing the frequency range of capture for a radio synchronization system.
It is yet another objective of the present invention to provide a novel system for frequency synchronization that does not require complex signaling processing.
It is still another object of the present invention to provide a novel method of frequency synchronization between a hub radio and remote radios in a BWA system.
It is yet still another object of the present invention to provide a novel system for reducing the time required to synchronize radios.
It is still another object of the present invention to provide a novel method of synchronizing a remote radio with a hub radio as a function of the hub radio""s frequency tolerance.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.